Our present findings indicate a sex difference in systolic blood-pressure (BP) in the rat which emerges several weeks after puberty. Males have higher BP than females. It is proposed to study the role of gonadal hormones via the use of gonadectomy and replacement therapy, both during development and in adult life, in the etiology of these sex differences (Note: preliminary results indicate that postnatal testosterone produces a modest increase in BP in adult life). Other recent findings indicate significant differences in peripheral norepinephrine levels between male and female rats. To elucidate these findings it is proposed to carry out more detailed studies of CA biochemistry in males and females. In addition, to discover if gonadal hormones mediate sex differences in ANS function via their effect on the peripheral sympathetic system the dependence of sex differences in BP on (a) differences in adrenal function and/or (b) differences in the function of the peripheral sympathetic system, will be separately investigated via adrenalectomy and chemical sympathectomy.